A parsimonious approach for large-scale tracer test interpretation

Abstract

Dye tracing is an efficient method for spring watershed delineation, but is also used in surface waters to assess pollution migration over several kilometers. The aim of this study is to develop a simple and parsimonious approach that accounts for a linear relationship between dispersivity and scale that could be used for the simulation of large-scale transport processes in aquifers. The analysis of 583 tracer recoveries is used to validate an inverse relationship between arrival time and peak concentration, which is shown to be a consequence of the linear relationship between dispersivity and scale. These results show that the tracer displacement through a given tracing system can be characterized at a large scale by a constant Peclet number. This interpretation is used to propose a new approach for tracer test design based on the analytical expression of the peak/time factor. It is also used for Peclet number assessment and simulation of the whole tracer residence-time distribution using a new method based on the ratio between the mode of the residence time distribution (hmod) and the corresponding time from injection (tmod), which is called the hmod/tmod method. This methodology is applied to two tracer tests carried out in a karst aquifer over 13 km between the same injection and detection points under distinct hydrological conditions. These results found practical applications in generalizing tracer test results to various flow conditions, or guiding the parameterization of physically-based vulnerability mapping methods.